1. Field of the Invention
The present invention relates generally to gelled fracturing fluids of the type used in well bore operations and particularly to a method for producing a gradual reduction in the viscosity of a gelled fracturing fluid through the use of chemical agents incorporated in the gelled fluid.
2. Description of the Prior Art
During hydraulic fracturing, a sand laden fluid is injected into a well bore under high pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation which generally continues to grow during pumping. The treatment design generally requires the fluid to reach maximum viscosity as it enters the fracture which affects the fracture length and width. This viscosity is normally obtained by the gellation of suitable polymers, such a suitable polysaccharide. The gelled fluid can be accompanied by a propping agent which results in placement of the propping agent within the fracture thus produced. The proppant remains in the produced fracture to prevent the complete closure of the fracture and to form a conductive channel extending from the well bore into the formation being treated once the fracturing fluid is recovered.
The recovery of the fracturing fluid is accomplished by reducing the viscosity of the fluid to a low value such that it flows naturally from the formation under the influence of formation fluids. This viscosity reduction or conversion is referred to as "breaking" and can be accomplished by incorporating chemical agents, referred to as breakers, into the initial gel.
In addition to the importance of providing a breaking mechanism for the gelled fluid to facilitate recovery of the fluid and resume production, the timing of the break is also of great importance. Gels which break prematurely can cause suspended proppant material to settle out of the gel before being introduced a sufficient distance into the produced fracture. Premature breaking can also result in a premature reduction in the fluid viscosity resulting in a less than desirable fracture width in the fracture being created.
On the other hand, gelled fluids which break too slowly can cause slow recovery of the fracturing fluid from the produced fracture with attendant delay in resuming the production of formation fluids. Additional problems can result, such as the tendency of proppant to become dislodged from the fracture, resulting in at least partial closing and decreased efficiency of the fracturing operation.
For purposes of the present application, premature breaking will be understood to mean that the gel viscosity becomes diminished to an undesirable extent before all of the fluid is introduced into the formation to be fractured. Thus, to be satisfactory, gel viscosity should remain in the range from about 50 to 75% of the initial viscosity of the gel for at least two hours of exposure to the expected operating temperature.
Optimally, the fracturing gel will begin to break when the pumping operations are concluded. For practical purposes, the gel should be completely broken within about 24 hours after completion of the fracturing treatment. A completely broken gel will be taken to mean one that can be flushed from the formation by the flowing formation fluids or that can be recovered by a swabbing operation. In the 14 laboratory setting, a completely broken, non-crosslinked gel is one whose viscosity is about 10 centipoises or less as measured on a Model 35 Fann viscometer at 300 rpm or less than 100 centipoises by Brookfield viscometer.
By way of comparison, certain gels, such as those based upon guar polymers, undergo a natural break without the intervention of chemical additives. The breaking time for such gelled fluids is excessive, being somewhere in the range from greater than 24 hours to about two weeks at an exposure temperature exceeding 80.degree. F. Accordingly, to decrease the break time of gels used in fracturing, chemical agents are incorporated into the gel and become a part of the gel itself. Typically, these agents are either oxidants or enzymes which operate to degrade the polymeric gel structure.
However, obtaining controlled breaks using oxidants or enzymes, especially at lower temperatures, has proved to be difficult. At temperatures ranging from ambient to 130.degree. F, the common oxidants are ineffective, requiring either higher temperatures to cause homolytic cleavage of the peroxide linkage or a coreactant to initiate cleavage. The enzymes, although effective at these lower temperatures, are sensitive to pH. Enzymatic activity rapidly declines after exceeding pH 8.0 and denatures above pH 9.0. In the case of borate crosslinked guar gels, the gels are also pH dependant requiring pH in excess of 8.0 to initiate gellation. As the pH is increased, the resulting gel becomes stronger, often requiring less borate crosslinker. Normally, these gels are buffered to maintain a pH range of 8.2 to 8.5 to ensure both gellation and enzymatic degradation. This technique requires high concentrations of both borate and enzyme. Unfortunately, while ensuring good breaks, the initial gel stability is also weakened. The determination of the optimum enzyme concentration is a compromise between initial gel stability and an adequate break.
The present invention has as its object to provide a break mechanism for a gelled fracturing fluid which yields high initial viscosity with little change during pumping but which produces a rapid break in the gel after pumping is completed to allow immediate recovery of the fluid from the formation.
Another object of the invention is to provide a gel system for a well fracturing operation which will completely break within about 24 hours but which will maintain a satisfactory viscosity at operating temperatures for a time sufficient to complete the fracturing treatment.